2022ApJ...936...75L

Changing-look active galactic nucleus NGC 4151, which has attracted a lot of attention, is undergoing the second dramatic outburst stage in its evolutionary history. To investigate the geometry and kinematics of the broad-line region (BLR), and measure the mass of supermassive black hole in NGC 4151, we perform a 7 month photometric and spectroscopic monitoring program in 2020-2021, using the 2.4 m telescope at Lijiang Observatory. We successfully measure the time lags of the responses from broad Hα, Hβ, Hγ, He I, and He II emission lines to continuum variation, which are ${7.63}_{-2.62}^{+1.85}$, ${6.21}_{-1.13}^{+1.41}$, ${5.67}_{-1.94}^{+1.65}$, ${1.59}_{-1.11}^{+0.86}$, and ${0.46}_{-1.06}^{+1.22}$ days, respectively, following radial stratification. The ratios of time lags among these lines are 1.23: 1.00: 0.91: 0.26: 0.07. We find that the continuum lag between the ultraviolet and optical bands can significantly affect the lag measurements of He I and He II. Virial and infalling gas motions coexist in this campaign, which is different from previous results, implying the evolutionary kinematics of BLR. Based on our measurements and previous ones in the literature, we confirm that the BLR of NGC 4151 is basically virialized. Finally, we compute the black hole mass through multiple lines, and the measurement from Hβ to be ${3.94}_{-0.72}^{+0.90}\times {10}^{7}{M}_{\odot }$, which is consistent with previous results. The corresponding accretion rate is ${0.02}_{-0.01}^{+0.01}{L}_{\mathrm{Edd}}{c}^{-2}$, implying a sub-Eddington accretor.